The present invention relates to a method for manufacturing a thin-film magnetic head equipped with a spin valve magnetoresistive effect (SVMR) element utilizing the giant magnetoresistive effect (GMR), used for a hard disc drive (HDD) unit.
Recently, thin-film magnetic heads with magnetoresistive effect (MR) read elements based on spin valve effect of GMR characteristics are proposed (U.S. Pat. Nos. 5,206,590 and 5,422,571) in order to satisfy the requirement for ever increasing data storage densities in today""s magnetic storage systems like HDD units. The SVMR element includes first and second thin-film layers of a ferromagnetic material separated by a thin-film layer of non-magnetic and electrically conductive material, and an adjacent layer of anti-ferromagnetic material is formed in physical contact with the second ferromagnetic layer to provide exchange bias magnetic field by exchange coupling at the interface of the layers. The magnetization direction in the second ferromagnetic layer is constrained or maintained by the exchange coupling, hereinafter the second layer is called xe2x80x9cpinned layerxe2x80x9d. On the other hand the magnetization direction of the first ferromagnetic layer is free to rotate in response to an externally applied magnetic field, hereinafter the first layer is called xe2x80x9cfree layerxe2x80x9d. The direction of the magnetization in the free layer changes between parallel and anti-parallel against the direction of the magnetization in the pinned layer, and hence the magneto-resistance greatly changes and GMR characteristics are obtained.
The output characteristic of the SVMR element depends upon the angular difference of magnetization between the free and pinned ferromagnetic layers. The direction of the magnetization of the free layer is free to rotate in accordance with an external magnetic field. That of the pinned layer is fixed to a specific direction (called as xe2x80x9cpinned directionxe2x80x9d) by the exchange coupling between this layer and adjacently formed anti-ferromagnetic layer.
In order to provide the exchange coupling between the pinned and anti-ferromagnetic layers, a process of temperature annealing under an external magnetic field with a specific direction (pin annealing or pin anneal process) is implemented. The pin annealing is done by elevating the temperature up to the Neel point at which magnetic regulation in the anti-ferromagnetic layer will be lost, and thereafter cooling down under application of magnetic field toward a desired magnetization direction.
In this kind of SVMR element, the direction of the magnetization in the pinned layer may change in some cases by various reasons. If the direction of the magnetization changes, the angular difference between the pinned and free layers changes too and therefore the output characteristic also changes. Consequently, controlling the direction of the magnetization in the pinned layer to a correct direction is very important.
However, the various characteristics of the SVMR element may be changed under actual high temperature operation of a HDD unit, even if the pin anneal processing is properly implemented. This change is caused by magnetic anisotropy change in the free layer due to the high temperature stress during operation of the HDD unit and due to the magnetic field by a hard magnet layer used for giving a bias magnetic field to the free layer.
The detail of this phenomenon is as follows.
(1) During fabricating process of the SVMR element, the free layer is deposited under application of magnetic field toward the track width direction. Thus, axis of easy magnetization of the free layer orients to the track width direction.
(2) The pin anneal process is done under application of magnetic field toward the pinned direction which is perpendicular to the easy magnetization axis of the free layer. Thus, the magnetic anisotropy of the free layer after the pin annealing may be weakened in comparison with that just after the deposition of the free layer, or the easy magnetization axis and the hard magnetization axis of the free layer may be reversed with each other.
(3) When the magnetic head with such a SVMR element is used under the state of high temperature, the magnetic anisotropy of the free layer changes again in accordance with magnetic field toward the track width direction from the hard magnet so that the axis of easy magnetization of the free layer gradually orients the track width direction.
(4) When actually using the magnetic head, magnetic field from the magnetic disk will be applied to the free layer in the direction perpendicular to the track width direction. Therefore, the change of the easy magnetization axis of the free layer toward the track width direction as described in
(3) results that any magnetization change in the free layer becomes difficult and that the reproduction output from the magnetic head lowers or deteriorates.
Thus, the magnetic anisotropy of the free layer immediately after the pin anneal process changes as the magnetic head is actually used, and as a result degradation of the head reproduction output and degradation of the symmetry of output wave may arise.
In order to reduce degradation which may be occurred at the time of such actual use, a performing of a free layer annealing wherein the SVMR element is additionally heated under application of magnetic field toward the track width direction so as to strongly fix the easy magnetization axis of the free layer in the track width direction has been proposed in Japanese patent unexamined publication No.10-223942.
However, if such the free layer annealing is performed, the pinned direction of the pinned layer may change due to the heat treatment under application of the magnetic field toward the track width direction and lowering of the head reproduction output may be brought as a result.
The detail of this degradation phenomenon is as follows.
(a) The pinned direction of the magnetization in the pinned layer is different from that of the magnetic domain control field that is generated by the hard magnet (track width direction). And hence the direction of the magnetization of the pinned layer which is contacted with the anti-ferromagnetic layer is slightly rotated toward the direction of the magnetic domain control field (hereinafter the direction of the magnetization of the pinned layer is expressed as xcex8p).
(b) In the anti-ferromagnetic material layer, the Neel point temperature differs from location to location inside the layer from macroscopic point of view, and it is distributed in a certain range of temperature. Even if the temperature is less than the xe2x80x9cbulkxe2x80x9d Neel point (average Neel point), there could be small area whose micro Neel point temperature is low and where the exchange coupling with the pinned layer disappears.
(c) When such SVMR element is operated at a high temperature T, which is equal to or less than the blocking temperature at which the exchange couplings of all microscopic areas disappear, and then cooled down to usual room temperature, some microscopic area whose Neel temperatures are less than T is effectively annealed again and the direction of the magnetization is rotated to xcex8p.
(d) The total amount of the xcex8p rotated area by the temperature cycle determines the magnetic structure of the anti-ferromagnetic layer and also the new direction of the magnetization of the pinned layer.
As stated in the above paragraph, performing of the free layer annealing may cause a change of the pinned direction in the pinned layer, and the electrical output characteristics of the SVMR element are degraded in signal levels, and waveform symmetry.
Hereinafter, the degradation of the output characteristics of the SVMR element due to the rotation of the pinned direction will be described with reference to drawings.
The SVMR element operates by detecting change in its electrical resistance depending upon an angle between directions of magnetization in the pinned and free layers as aforementioned. The electrical resistance R is expressed by R=(1xe2x88x92cos xcex8)/2+xcex1, where xcex8 is the angle between directions of magnetization in the pinned and free layers and xcex1 is an electrical resistance (Rs) when the magnetization directions in the pinned and free layers are in parallel (xcex8=0 degree) as illustrated in FIG. 1a. When the magnetization directions in the pinned and free layers are in anti-parallel (xcex8=180 degrees) as illustrated in FIG. 1b, the electrical resistance becomes R=1+xcex1. Also, when the magnetization directions in the pinned and free layers are orthogonal (xcex8=90 degrees) as illustrated in FIG. 1c, the electrical resistance becomes R=1/2+xcex1.
As illustrated in FIG. 2, the SVMR element produces output voltage in response to the change in magnetization direction of the free layer caused by application of changing leakage magnetic field from the magnetic recording medium. Suppose that the direction of magnetization in the free layer rotates by +20 degrees (first magnetization state of the free layer) and by xe2x88x9220 degrees (second magnetization state of the free layer) due to the leakage magnetic field from the magnetic recording medium. If the pinned direction is normal, the resistance value across the SVMR element at the first magnetization state RF1 is RF1=(1xe2x88x92cos 70xc2x0)/2=0.329 and the resistance value across the SVMR element at the second magnetization state RF2 is RF2=(1xe2x88x92cos 110xc2x0)/2=0.671 as shown in FIG. 3a. Thus, the difference xcex94R becomes as xcex94R=RF2xe2x88x92RF1=0.342. Whereas, if the pinned direction rotates by 20 degrees from the normal direction, the resistance value across the SVMR element at the first magnetization state RF1 is RF1=(1xe2x88x92cos 50xc2x0)/2=0.178 and the resistance value across the SVMR element at the second magnetization state RF2 is RF2=(1xe2x88x92cos 90xc2x0)/2=0.500 as shown in FIG. 3b. Thus, the difference xcex94R becomes as xcex94R=RF2xe2x88x92RF1=0.322. Therefore, 20 degrees rotation of the pinned direction results degradation of 5.8% in the SVMR element output.
It will be understood from the above-description that output degradation of a SVMR element contains both degradation with time due to the applied heat and magnetic field during the actual use and first stage degradation due to change in the pinned direction caused by the free layer annealing. However, no manufacturing of a thin-film magnetic head had been performed so as to reduce the both degradations. For this reason, it was quite difficult to totally reduce output degradation.
It is therefore an object of the present invention to provide a manufacturing method of a thin-film magnetic head, whereby degradation of output of a SVMR element can be reduced certainly.
The present invention relates to a method of manufacturing a thin-film magnetic head with a SVMR element which includes first and second layers of a ferromagnetic material (free and pinned layers) separated by a layer of non-magnetic electrically conductive material, and a layer of anti-ferromagnetic material formed in physical contact with the second ferromagnetic material layer. According to the present invention, the method has a first temperature annealing (pin annealing) step of annealing the SVMR element under application of magnetic field to provide exchange coupling between the second ferromagnetic material layer and the anti-ferromagnetic material layer so that the second ferromagnetic material layer is pinned toward a predetermined direction, and a second temperature annealing (free layer annealing) step of annealing the SVMR element so that axis of easy magnetization of the first ferromagnetic material layer orients a direction substantially perpendicular to the predetermined direction. The free layer annealing is performed at a temperature lower than 150xc2x0 C.
If annealing temperature in the free layer annealing is made high, although the degradation with lapse of time due to heat and magnetic field during the actual usage of the SVMR element can be reduced, the first stage degradation due to the rotation of the pinned direction in the pinned layer will increase conversely. However, it is enabled to reduce certainly the total degradation which is the sum of the first stage degradation and the degradation with lapse of time if the free layer annealing is performed at a temperature lower than 150xc2x0 C. as the present invention. Consequently, the large enhancement of a yield can be expected.
It is preferred that the free layer annealing is performed at a temperature equal to or higher than 100xc2x0 C.
It is also preferred that the free layer annealing is performed under application of magnetic field in a direction substantially perpendicular to the predetermined direction. In this case, it is more preferred that the free layer annealing be performed under application of external magnetic field of 500 Oe or less.
The free layer annealing is preferably performed just after the pin annealing, or at the same time as the pin annealing.
It is preferred that the method further has a step of forming of bias providing means for applying magnetic domain control field to the free layer, and that the free layer annealing is performed under no application of external magnetic field after forming the bias providing means.
The free layer annealing is preferably performed just after the pin annealing.
It is also preferred that the free layer annealing is performed for 1-10 hours.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.